Cationic surfactants used as antimicrobial agent is gaining increasing acceptance recently. One such antimicrobial is Nα-Lauroyl-L-arginine ethyl ester monohydrochloride (LAE). This antimicrobial, LAE, is approved as a food ingredient in the United States [1] and a few other countries. It is also an approved compound for the cosmetic industry and hair conditioners [2]. Such acceptance of this ingredient reflects on its safety profile. Yet the product remains encumbered by impurities that reflect negatively upon its antimicrobial properties and its applications in some areas.
The prior art patents [3-8] describe the preparation of cationic surfactant compounds. US20110077423 and US20100152480 review earlier patent literature and advances the process chemistry by condensing acid halide of fatty acid with amino acid ester derivative in aqueous medium while controlling temperature and pH by eliminating use of expensive coupling agent dicyclohexylcarbodiimide (DCC) and anhydrous reaction conditions. The process described in these patents, though an advancement above the prior art, is encumbered with generation of undesirable impurities. Further, the process requires both temperature and pH control in narrow ranges. Though a comparatively pure product is claimed but the impurity profile remains the same. The described process has inherent drawback of forming undesirable impurities due to use of hydrolytic aqueous condition and harsh neutralizing base (at the concentration of sodium hydroxide being used) which at initial contact affords high unintended localized pH.
The products flowing out of the prior art processes contain several impurities. These impurities, though not desirable, were recognized in earlier specifications of the product. Following is typical impurity profile reported [9].
PURITYTotal ash (Vol. 4)Not more than 2% (700°)Water (Vol. 4)Not more than 5%. Determine by the methodsdescribed in Volume 4 under “GeneralMethods, Water Determination (Karl FischerMethod)”.Nα-Lauroyl-L-arginineNot more than 3%Lauric acidNot more than 5%Ethyl laurateNot more than 3%L-Arginine•HClNot more than 1%Ethyl arginate•2HClNot more than 1%
Recently better yield of 98% w/w has been reported in US20110077423. This percentage yield is presumed to be with respect to the input weight of 47.1 gm of L-arginine ethyl ester dihydrochloride (in Example 1 of the referred patent) which when converted to the theoretical yield from the reaction calculates to ˜64% on molar basis. Further, the described workup of the reaction mass indicating 98% w/w yield (64% molar basis) is inclusive of the unavoidable impurities due to the inherent process conditions. Hence, for a novice in this art the improved yield numbers have to be viewed with caution as they are w/w and inclusive of the impurities formed in the reaction conditions. Thus the yield of the product reported in the prior art is low compared to the yield reported by the inventors on molar basis.